The present invention relates to a color cathode ray tube, and particularly to a color cathode ray tube employing an in-line type electron gun configured to project three electron beams in a line toward a phosphor screen.
Color cathode ray tubes are widely used as display devices for television receivers and terminal monitors of information equipment represented by personal computers.
FIG. 4 is a sectional view illustrating a configuration example of a color cathode ray tube to which the present invention is applied. Reference numeral 20 indicates a panel portion for forming a screen; 21 is a neck portion; 22 is a funnel portion; 23 is a phosphor screen; 24 is a shadow mask; 25 is a mask frame; 26 is a magnetic shield; 27 is a shadow mask suspension mechanism; 28 is an in-line type electron gun; 29 is a deflection device; 30 is a magnetic correction device for beam centering and color purity adjustment; 31 is a getter; 32 is an internal conductive layer; 33 is a tension band; 34 is stem pins for supplying a video signal and various voltages to the electron gun; 35 is an anode button; and 36 is a contact spring for supplying a high anode electrode voltage to the electron gun.
In the color cathode ray tube of this type, an evacuated envelope is composed of the panel portion 20, the neck portion 21, and the funnel portion 22 connecting the panel portion 20 to the neck portion 21, and a portion connecting the panel portion 20 and the funnel portion 22 is tightly wound by the tension band 33 for implosion protection.
The phosphor screen 23 coated with phosphors of three colors, red, green, and blue in a stripe pattern or a dot pattern is formed on the inner surface of the panel portion 20, to form a viewing screen.
The in-line type electron gun 28 for projecting three electron beams in a line in a horizontal plane is housed in the neck portion 21. The shadow mask 24 having a multi-apertured thin plate or parallel wires and serving as a color selection electrode is fixed to the mask frame 25 and is suspended from the inner wall of the panel portion 20 by the shadow mask suspension mechanism 27 at a position close to the phosphor screen 23 formed on the inner surface of the panel portion 20.
Characters Bc and Bs.times.2 indicate three electron beams. The deflection device 29 for deflecting the electron beams in the horizontal and vertical directions to scan the electron beams two-dimensionally on the phosphor screen is mounted in a transition region between the funnel portion 22 and the neck portion 21.
An anode voltage is applied from the anode button 35 formed in the funnel portion 22 to the electron gun 28 by way of an internal conductive coating 32 and the contact spring 36. Three electron beams, one center beam Bc and two side beams Bs, projected from the electron gun 28 in a line are deflected in the horizontal and vertical directions through horizontal and vertical deflection magnetic fields generated by the deflection yoke 29, being subjected to color selection by the shadow mask 24, and impinge upon respective phosphors forming the phosphor screen 23, to thus form a color image on the panel portion 20.
The magnetic correction device 30 is mounted around the neck portion 21 for centering adjustment for landing three electron beams on the center of the screen and color purity adjustment for landing three electron beams on respective proper phosphors.
The in-line type electron gun housed in neck portion 21, although it may be of any one of known various types, generally includes a triode section having three cathodes, a first grid electrode, and a second grid electrode; a focus electrode for accelerating and focusing electron beams; and an anode forming a main lens in cooperation with the focus electrode.
On example of such an in-line type electron gun is disclosed in Japanese Patent Laid-open No. Sho 58-103752.
FIG. 5 is a schematic sectional view illustrating a configuration of the in-line type electron gun disclosed in the above document.
FIGS. 6A and 6B are schematic sectional views showing essential portions of the electron gun shown in FIG. 5, FIG. 6A is the sectional view taken on line VIA--VIA, and FIG. 6B is the sectional view taken on line VIB--VIB.
In FIG. 5, reference numerals 2R, 2G and 2B indicate cathodes for red, green and blue beams, respectively; 3 is a first grid electrode as serving a control grid; 4 is a second grid electrode serving as an accelerating electrode. In FIG. 5 and FIGS. 6A and 6B, reference numeral 5 indicates a third electrode serving as a focus electrode; 5a is a single opening in a fifth electrode; 6 is a plate electrode provided within the focus electrode; 7 is an anode; 7a is a single opening in the anode; and 8 is a plate electrode provided within the anode electrode.
Each of the focus electrode 5 and the anode electrode 6 has a transverse cross-section of an approximately ellipse with its major axis in the in-line direction or a horizontally elongated rectangle with its corners rounded, has a single opening allowing three electron beams to pass therethrough at least at its end face facing the end face of the other (a main lens-forming end face).
As shown in FIG. 6A, the plate electrode 6 disposed in the focus electrode 5 has an elliptical center beam aperture with its major axis vertical and two side beam apertures having a pair of oppositely disposed semi-circles or semi-ellipses of same or different radii. As shown in FIG. 6B, the plate electrode 8 disposed in the anode 7 has an elliptical center beam aperture with its major axis vertical and two semi-elliptical cutouts formed on both the sides of the center beam aperture.
The operation of the in-line type electron gun having the above configuration will be described.
Thermoelectrons emitted from the three cathodes 2R, 2G and 2B heated by heaters (not shown) are attracted toward the first grid electrode (or control grid) 3 by a positive voltage of 400 to 1000 V applied to the second grid electrode (or acceleration electrode) 4, to form three electron beams Bs, Bc and Bs.
The three electron beams Bs, Bc and Bs pass through an aperture in the first grid electrode (control grid) 3 and then through an aperture in the second grid electrode (accelerating electrode) 4, are accelerated by positive voltages applied to the third electrode (focus electrode) 5 and the anode electrode 7 and enter the main lens.
Here, before entering the main lens, the electron beams are slightly focused by a prefocus lens formed between the accelerating electrode 4 and the focus electrode 5 supplied with a low voltage of about 5 to 10 kV.
A high voltage of about 20 to 35 kV is applied to the anode 7 constituting the main lens. The electron beams are focused on the phosphor screen by the main lens formed by a potential difference between the focus electrode 5 and the anode electrode 7, to form beam spots on the screen.
In a high definition color cathode ray tube employing a prior art in-line electron gun having the above configuration, particularly used for an information terminal such as a computer, if a conventional value of 29.1 mm is adopted for the outside diameter of a neck portion, a deflection power consumption in a deflection device increases with increase in deflection frequency due to the number of scanning lines increased for a high definition display, and therefore the neck portion is desired smaller.
The reduction in the outside diameter of the neck portion, however, presents a problem in that the main lens diameter of the electron gun is reduced, to increase the beam spot diameter on the phosphor screen, thereby degrading the resolution.
To solve the above problem, there is proposed a technique, for example, in Japanese Patent Laid-open No. Hei 5-325826. In this technique, an outside diameter T of a neck portion containing an in-line electron gun and a center-to-center spacing S between adjacent electron beams are set to satisfy a relationship of 2S+14.6 mm.ltoreq.T.ltoreq.25.3 mm and 4.1 mm.ltoreq.S, and a low power consumption and a small beam spot diameter are made compatible with each other.
The above-described technique, however, has the following disadvantage. When a low light transmittance panel (dark tainted panel) is employed for a cathode ray tube, the brightness to be produced by the phosphor needs to be increased to make up for reduction in the viewing screen brightness caused by the a low light transmittance panel. This in turn requires the beam current to be increased to raise the brightness produced by the phosphors, and an increase in the beam increases the beam spot diameter, failing to retain the focus characteristics.